1. Field of the Invention
The present invention relates to a hydraulic system in which pressurized fluid from a source is applied in a controlled manner to a plurality of hydraulic actuators that produce movement of different components on a machine, and in particular to devices that determine which of the hydraulic actuators are to be operable when insufficient fluid is available from the source to operate all the hydraulic actuators.
2. Description of the Related Art
Modern aircraft employ hydraulic systems to operate various mechanical components, such as ailerons, elevators and the rudder which are parts of the flight control system, as well as doors and landing gear. One or more hydraulic pumps furnish pressurized fluid to a plurality of valve assemblies, each controlling the application of the pressurized fluid to a hydraulic actuator that moves a component on the aircraft. A given valve may be mechanically operated by a member of the flight crew or may be electrically operated either by a crew member or by an electronic controller.
Normally, the pumps furnish sufficient hydraulic fluid so that as many of the hydraulic actuators can be operated simultaneously as is necessary. However, conditions occur in which the pumps are incapable of furnishing enough hydraulic fluid to operate all the desired actuators at the same time. At those times, it is desirable that the hydraulic actuators associated with flight control be able to operate as normally as possible. Therefore, when a limited amount of hydraulic fluid is available, that fluid should be allocated to the flight controls on a priority basis before being made available to less critical functions.
For that purpose, a priority control valve was incorporated in the hydraulic system to enable flight control actuators to operate as normally as possible, while limiting fluid flow to other less critical hydraulic actuators. Prior priority control valves sometimes exhibited an adverse condition commonly called “thrashing.” That condition occurred when the priority control valve attempted to close in response to the flow to the secondary actuators that caused a reduction in pressure to the primary actuators. The closing action resulted in an increase of the pressure for the flight control actuators to which the priority control valve reacted by attempting to reopen. It is possible for the response time of the hydraulic system to be such that this open-close-open cycle became a continuous, resonant cycling that was harmful to the system.
As a consequence, it is desirable to provide a device that automatically recognizes when insufficient hydraulic fluid is available for operating all the hydraulic actuators and allocating the available fluid only to high priority actuators. It is further desired to reduce or eliminate the thrashing condition encountered with previous priority control valves.